use strict;
use warnings;

use XML::Parser;
use XML::Twig;

my $twig=XML::Twig->new(
                        twig_roots=>{
                                                                     indd_document=>\&update_root,

                                    'document/header'=>\&update_header,
                                    },
                        );
$twig->parsefile( $xmlfile);
open(FH,">output.xml") or die "cannot open output.xml: $!";
my $temp = $twig->toString;
print FH $temp;
close (FH);

$twig->purge;
}
sub update_root
{
my ($t, $root) = @_;
$root->insert_new_elt(first_child => header => {type=>"layout"});
$root->set_tag( 'document');
}

sub update_header
{
my ($t, $header)=@_;
$header->insert_new_elt(first_child => headerfield => {type=>"layout"});
}


##</code><code>##

<?xml version="1.0"?>
<indd_document>
<para>
</para>
</indd_document>

##</code><code>##

<?xml version="1.0"?>
<indd_document>
<header>
<headerchild name=""/>
<headerchild name=""/>
</header>
<para>
</para>
</indd_document>